Since the radar level gauging was developed as a commercial product in the 1970's and 1980's, frequency modulated continuous wave (FMCW) has been the dominating measuring principle for high accuracy applications. An FMCW measurement comprises transmitting into the tank a signal which is swept over a frequency range in the order of a few GHz. For example, the signal can be in the range 24-27 GHz, or 9-10.5 GHz. The transmitted signal is reflected by the surface of the contents in the tank (or by any other impedance transition) and an echo signal, which has been delayed a certain time, is returned to the gauge. The echo signal is mixed with the transmitted signal to generate a mixer signal, having a frequency equal to the frequency change of the transmitted signal that has taken place during the time delay. If a linear sweep is used, this difference frequency, also referred to as an intermediate frequency (IF), is proportional to the distance to the reflecting surface. The mixer signal is often referred to as an IF signal.
More recently, the FMCW principle has been improved, and today typically involves transmitting not a continuous sweep but a signal with stepped frequency with practically constant amplitude. When the transmitted and received signals are mixed, each frequency step will provide one constant piece of a piecewise constant IF signal.
The distance to the reflecting surface is then determined using the frequency of the IF signal in a similar way as in a conventional FMCW system. Typical values can be 200-300 IF periods at 30 m distance divided in 1000-1500 steps.
It is noted that also a continuous IF signal, resulting from a continuous frequency sweep, may be sampled in order to allow digital processing.
Although highly accurate, conventional FMCW systems (continuous as well as stepped) are relatively power hungry, making them less suitable for applications where power is limited. Examples of such applications include field devices powered by a two-wire interface, such as a 4-20 mA loop, and wireless devices powered by an internal power source (e.g. a battery or a solar cell).
The main power consumer is the microwave module, which, due to the requirements on frequency accuracy, requires relatively high power to generate and emit the microwave energy during each sweep. Between sweeps suitable means can be used to store power, so that a lower average power can used to power the microwave module for the duration of the sweep. However, due to space limitations and intrinsic safety (IS) requirements, such power storage capacity is severely limited. Therefore, it is crucial to limit the active period of the microwave module, i.e. to limit the duration of the sweep. Further, it is desirable to limit the sampling rate, in order to reduce the power consumption in the analogue signal processing and the A/D conversion. Finally, from a performance point of view, it is advantageous to have a wide bandwidth, providing a more robust measurement.
In a radar level gauge system using a time sequence of discrete and mutually different frequencies as the transmit signal, the active time of the microwave module can be reduced by reducing the number of frequencies in the transmit signal.
However, a reduced number of frequencies over a maintained bandwidth may result in a distorted IF signal, which may in turn introduce false echoes that could be mistaken for the surface echo.
Accordingly, reducing the power consumption of the radar level gauge system by reducing the number of frequencies in the transmit signal may result in a reduced reliability of the filling level determination.